A spark erosion profiling machine tool may comprise a fixed workpiece holding means and a tool head which comprises an elongate tool holding member and means for linearly moving the tool holding member in the direction of its length so that a tool attached to the forward end of the member moves in relation to the workpiece and executes the machining process.
The accuracy of movement of the tool has an important effect on the accuracy, stability and quality of the machining process and so the tool holding member must be supported in the tool head of the machine in such a way that its linear movement is guided with a high degree of accuracy. In addition, the member should have a high degree of rigidity and resistance to torsional and bending moments which may be exerted on it during the machining process, for example due to the interaction of the tool and the flushing fluid which is used in spark erosion machining, especially when the movement of the tool includes a reciprocating component to assist the flushing effect.
Tool holding members of the relevant kind have hitherto generally been made of circular cross-section and have either been directly mounted in say a hydraulic cylinder forming the means for moving the member, or have been guided by means of linear bearings comprising live rolling elements. To resist torsional moments, additional structures have been used, for example a round or prismatic guide member mounted off-centre in relation to the axis of the holding member, or chamfering of the holding member or some part thereof and the provision of a correspondingly shaped fixed part which co-operates with the chamfered part to resist torsion.
Some tool holders comprise two or more round guide members which automatically resist torsional moments applied thereto. This, however, involves additional cost and problems regarding aligning the assembly.
There are known prismatic spark erosion tool holding members of various cross-sectional forms which are mounted in plain bearings or in live roller bearings, for example needle slide bearings. Plain bearings are subject to known problems, for example they can be made or adjusted to have very small clearances in which case the friction becomes high or, if the clearances are made larger to reduce friction, the play and "back-lash" of the tool holding member becomes unacceptably high for the present purpose. In addition, such bearings wear too quickly and are subject to "stick-slip" during slow movement (which is often required during spark erosion machining, for example for finishing operations). Live roller bearings are much better than plain bearings in regard to the above matters, i.e. they have much lower friction ratios for a given rigidity of the tool holding member. However they are attended by the additional problem that, due to the fact that the rolling elements are "live", i.e. they move with the tool holder, the length of the housing or the like forming the counter member of the assembly, i.e. the member between which and the tool holding member the rolling elements are interposed, has to be substantial in relation to the stroke of the tool holding member. In addition, only a limited and constantly varying distance between bearings is obtained which puts an upper limit on the rigidity of the tool holding member in the bearing, which rigidity is in any case not constant for all positions of the tool holding member over the length of its stroke.